1. Field of the Invention
The invention relates to the field of materials technology. It concerns a process for applying a metallic adhesion layer for thermally sprayed ceramic thermal barrier coatings (TBC) to metallic components and a metallic adhesion layer produced in accordance with this process.
2. Discussion of Background
Normally, metal and ceramic cannot be joined to one another owing to the different coefficients of thermal expansion.
It is known that, in order to solve this problem, a ductile intermediate layer is placed between the parts which are to be joined, which intermediate layer compensates for the differential expansions at different temperatures in an elastic-plastic manner (cf. W. J. Brindley, R. A. Miller: "TBCs for better engine efficiency", Nasa Lewis Research Center Cleveland, Advanced Materials & Progress 8/1989, pp. 29-33). These intermediate layers, which are designated as adhesion layers, are conventionally applied by means of known flame spraying processes, plasma spraying processes or detonation spraying processes. They make possible a metallurgical-mechanical bonding to the metallic component and a purely mechanical bonding of the likewise thermally sprayed ceramic layer to the adhesion layer, this joint exhibiting a pronounced sensitivity to impacts and thermal shocks.
Since the ceramic thermal barrier coatings protect the coated metallic components from harmful thermal stresses, it is important for a sufficient service life of the components that they be present without gaps. Components which have been coated in this manner are used in particular in the field of combustion technology, for example for combustion chamber parts or gas turbine blades.
The disadvantage of the metallic adhesion layers for ceramic thermal barrier coatings which have been produced to date is that they exhibit insufficient roughness and thus provide inadequate positive lock (undercuts), so that the layer thickness of the TBC layers is limited. Layer thicknesses of approx. 0.2 to 0.4 mm are known, layer thicknesses of about 0.3 mm being most commonly encountered. If they are thicker, the risk of flaking increases rapidly. If they are thinner, the heat insulating effect drops quickly. Although more recent developments have tended toward coarser adhesion layers (approx. 0.6 mm) being sprayed, the required positive lock is lacking.
A roughness (peak-valley height) which is typical for known metallic adhesion layers is about 30 .mu.m. It is impossible to spray the layers any more roughly, since the size of the powder particles to be melted is limited to approx. 10 to 50 .mu.m, depending on the coating process (different spraying temperatures and spraying speeds), and the liquid powder particles become smoothed out on striking the substrate (cf. B. Heine: "Thermisch gespritzte Schichten" Thermally sprayed layers!, Metall, Vol. 49, 1/1995, pp. 51-57).
An obvious remedy, by roughening to a coarser level by sandblasting or by changing the flame spraying parameters, is, however, limited. For example, although the layer thickness of the TBC ceramic layer can be increased by low-speed flame spraying, layers of this kind do not withstand any thermal shocks.
Rough threading or milling grooves into the surfaces to be coated, as are specified by B. Heine in the abovementioned article in order to promote adhesion with desired layer thickness of greater than 1 mm, are expensive and can be achieved only with great difficulty in the event of a complicated geometric shape of the workpiece.